1. Field of the Invention
The field of the present invention relates to a method and apparatus for multiple access communication.
2. Background
A variety of techniques are known for allowing multiple users to communicate with one or more fixed stations (i.e., base stations) by making use of shared communication resources. Examples of multiple access communication systems include, for example, cellular telephone networks and local wireless communication systems, such as wireless private branch exchange (PBX) networks. In such multiple access communication systems, transmissions from different sources may be distinguished in a variety of manners, such as on the basis of different frequencies, time slots, and/or codes, for example.
As used herein, a communication system in which transmissions are distinguished according to the transmission frequency may be referred to as a frequency division multiple access (FDMA) communication system. A communication system in which a forward link transmission over one frequency is paired with a reverse link transmission over a different frequency may be referred to as a frequency division duplex (FDD) communication system.
A communication system in which transmissions are distinguished according to the relative timing of the transmission (i.e., by use of time slots) may be referred to as a time division multiple access (TDMA) communication system. A communication system in which a forward link transmission during one time slot (or time segment) is paired with a reverse link transmission occurring during a different time slot (or time segment) may be referred to as a time division duplex (TDD) communication system. The DECT system is an example of a well known type of TDD communication system.
A communication system in which transmissions are distinguished according to which code is used to encode the transmission may be referred to as a code division multiple access (CDMA) communication system. In a CDMA communication system, the data to be transmitted is generally encoded in some fashion, in a manner which causes the signal to be xe2x80x9cspreadxe2x80x9d over a broader frequency range and also typically causes the signal power to decrease as the frequency bandwidth is spread. At the receiver, the signal is decoded, which causes it to be xe2x80x9cdespreadxe2x80x9d and allows the original data to be recovered. Distinct codes can be used to distinguish transmissions, thereby allowing multiple simultaneous communication, albeit over a broader frequency band and generally at a lower power level than xe2x80x9cnarrowbandxe2x80x9d FDMA or TDMA systems. Different users may thereby transmit simultaneously over the same frequency without necessarily interfering with one another.
Various xe2x80x9chybridxe2x80x9d communication systems incorporating aspects of more than one multiple access communication technique have been developed or proposed. For example, a GSM system may be viewed as a xe2x80x9chybridxe2x80x9d communication system utilizing aspects of both FDD and TDMA. In a GSM system, each base station is assigned a transmission frequency band and reception frequency band. The base station transmits to each of its mobile stations using a transmission frequency within its assigned frequency band, and the mobile stations transmit to the base station using a frequency within the base station""s reception frequency band. The transmissions to the user stations are sent in assigned time slots over the base station""s transmission frequency, and the transmissions from the user stations are sent in corresponding assigned time slots over the base station""s reception frequency.
While multiple access communication may be achieved using techniques of either FDMA, TDMA or CDMA, or certain variations (e.g., FDD or TDD) or combinations thereof, problems can occur if an equipment manufacturer or operator desires to migrate from one type of multiple access communication to a different type. This problem results from the fact that equipment manufactured specifically for any one type of multiple access communication system typically cannot be used with another type of multiple access system because of inherent differences in the nature of the communication techniques, leading to incompatibilities between the physical hardware as well as the communication protocols employed by the two communication systems. For example, a base station designed for TDD communication cannot be expected to communicate properly with an FDD handset, nor can it be expected that a TDD handset will communicate properly with a base station designed for FDD communication.
It may nevertheless be desired by equipment manufacturers or service providers to deploy or offer systems using different multiple access communication techniques or protocols, in order to serve different markets, geographical regions, or clientele, or for other reasons. However, to develop separate equipment for operation in different multiple access communication environments can substantially increase equipment design and manufacturing costs. Such a development process can also lead to the creation of different and incompatible protocols, which can require, for example, different types of backhaul service, leading to greater design expense to support the different backhaul formats and possibly duplicative base station controllers in the same local area, each servicing a different type of base station (i.e., FDD vs. TDD). Furthermore, an equipment manufacturer or service provider may desire to migrate from one type of multiple access communication and protocol to another type, without incurring substantial redesign costs.
It would therefore be advantageous to provide an apparatus and method allowing communication in more than one multiple access communication environment. It would further be advantageous to provide a method and apparatus for converting or adapting equipment from one type of multiple access communication service (e.g., TDD) to a different type (e.g., FDD).
The invention provides in certain aspects techniques for using or converting multiple access communication equipment to serve in a different multiple access communication environment.
In one embodiment, a base station within a communication system comprises two base station sub-units, preferably collocated, for performing virtual FDD communication. Each of the two base station sub-units comprises a base station transmitter and a base station receiver. The first base station sub-unit transmits base-to-user messages to user stations only during a first half of a repeating time frame, and receives user-to-base messages from the user stations only during a second half of the time frame. The second base station sub-unit is preferably collocated with the first base station sub-unit, and transmits base-to-user messages to user stations only during the second half of the time frame, while receiving user-to-base messages from the user stations only during the first half of said time frame. The two base station sub-units are preferably synchronized so as to maintain proper alignment of the time frame and of the time slots within the time frame.
In a second embodiment, a base station also comprises two base station sub-units. In the second embodiment, a time frame comprises a plurality of base transmit time slots defined with respect to a base transmit frequency band and a plurality of user transmit time slots defined with respect to a user transmit frequency band. The time frame is divided between the two base station sub-units such that the first base station sub-unit and second base station sub-unit each are assigned one half of the base transmit time slots and one half of the user transmit time slots. The base transmit time slots assigned to each base station sub-unit may form a contiguous block, or may alternate with one or more base transmit time slots assigned to the other base station sub-unit. Duplex communication channels are preferably defined by correlating a base transmit time slot with a user transmit time slot, with the base transmit time slots and user transmit time slot preferably separated by a sufficient amount of time to allow transmit/receive switching by a user station between the base transmit time slot and the user transmit time slot. Multiple time slots may be aggregated to a single user station in certain embodiments.
In another embodiment, a base station comprises a pair of modified TDD base station sub-units. One of the modified TDD base station sub-units is adapted to transmit continuously over a base transmit frequency band using its base station transmitter, while the other of the modified TDD base station sub-units is adapted to receive continuously over a user transmit frequency band using its base station receiver. A backhaul interface transmits information over a backhaul line from the modified TDD base station sub-unit that receives continuously, and transmits information from the backhaul line to the modified TDD base station sub-unit that transmits continuously, so as to support a plurality of duplex communication channels. The two modified TDD base station sub-units may, in one embodiment, pass appropriate synchronization and error correction information to one another over a signal interface.
In another embodiment, a TDD base station is adapted to support FDD communication. The TDD base station comprises a radio transceiver, an over-the-air controller, a memory buffer and backhaul interface. The over-the-air controller switches the transmit and receive operating frequency between a base transmit frequency band and a user transmit frequency band in accordance with a defined time slot communication pattern comprising base transmit time slots and user transmit time slots, and at the same time ensures that the base transmitter and base receiver are appropriately switched back and forth for connection with the base station antenna (or antennas). The modified TDD base station may toggle back and forth between base transmit time slots and user transmit time slots on a slot-by-slot basis, or else may switch the base transmit frequency band and user transmit frequency band after a predefined number of transmit time slots or user transmit time slots.
Further embodiments, modifications, variations and enhancements of the invention are also disclosed herein.